This invention relates to a positive acting resist material for use in the lithographic process of forming patterns in the manufacture of microelectronic devices such as, for example, semiconductor devices and bubble memory elements. The resist material is of the type comprised of an alkali soluble resin and a photosensitive agent.
In the manufacture of microelectronic devices represented by semiconductor devices by direct delineation of patterns with an ionizing radiation, desired patterns are first formed in a resist material coated on a substrate. The substrate is formed with alignment marks in advance. To form patterns in the predetermined locations of the resist layer on the substrate, the alignment marks are detected by scanning with a charged beam before the exposure and development operations. The resist layer formed with the patterns serves as a protecting layer in the subsequent operation for processing the underneath substrate or implanting ions into the substrate. Accordingly the formation of the resist patterns is a very important operation step in the entire manufacturing process.
At the step of forming such resist patterns, either a positive acting resist or a negative acting resist is selectively used depending on some factors such as the proportion of the areas to be exposed and the manner of processing the substrate. For example, in the cases of forming contact holes it is very favorable and is prevailing to employ a positive acting resist for forming resist patterns.
Conventional positive acting resists for use in forming patterns by direct delineation with electron beam are generally composed of a cresol-formaldehyde novolac resin and a photosensitive agent, which is usually either a quinonediazide compound or poly(2-methylpentene-1-sulfone). As is well known, these positive acting resists become insoluble by crosslinking reaction when exposed at high doses. This property of the positive acting resists raises a problem in connection with the aforementioned detection of the alignment marks, which is an indispensable operation step in the lithography process for forming patterns.
That is, the positive acting resist film undergoes crosslinking reaction and consequently turns into an insoluble gel in the areas around the alignment marks on the substrate since the scanning with a beam of charged particles for detecting the alignment marks is repeated many times, viz. nearly 50 times in most cases, on each substrate coated with the resist film. In the case of electron beam exposure it is desirable to keep the column conditions unchanged even at the times of detecting the alignment marks with a view to enhancing the precision of the lithography process. Accordingly, at each time of detecting the alignment marks the resist film is locally scanned with electron beam at the same exposure dose as in the exposure operation to form positive tone patterns. Therefore, gelling of the resist film in the repeatedly scanned areas is promoted and intensified. Since the gelled areas of the resist film can hardly be removed by ordinary solvents, it is usual to perform ashing with oxygen or dipping in a heated remover liquid comprising a strong acid such as nitric acid or sulfuric acid for removal of the gelled resist. However, unfavorable effects of such treatment on the substrate cannot be neglected.